I need to confirm whether this is a problem or not. Let me describe my concern.
I was traveling a lot for last 2 weeks (5000 km) across the Europe and during refueling I spotted following thing.

1. When fuel tank meter was at "red" on R letter, I could put 58 liters of diesel into the tank, when fuel pump stopped for the first time. Fuel computer was showing 0 km on tank when refueling. After refueling, fuel tank gauge doesn't align with "F" (1-2 mm away from F mark), and after 20-30km of motorway driving at 130km/h constant speed, it drops between 3/4 and F marks.
2. Same situation but this time, I'm trying to squeeze as much fuel as possible after first fuel pump stop. I'm able to put 10 liters of fuel more, which ends at 68-70 liters. This time fuel tank gauge goes 1mm past F mark and it will stay around this mark for 80-110km since refueling.

As far as I'm aware, A6 3.0 TDI are equipped with 80 liters fuel tank as standard? Mine car is 2005 - UK specs. If that's correct, why I'm able to fill 60 liters even the fuel gauge says the tank is dry?! I can fill up to 70 liters but fuel pump stops every 0.5-1 liter I tried to put above 60 liters mark?!

When my DIS says Zero Miles - I can only ever get 70 litres into my tank.

... I remember seeing a switch somewhere in VAG-COM saying 70 litre tank, and I am wondering if both yourself and I have the wrong setting, so DIS is telling us we need to refuel too early.

I need to check mine (which I won't be able to do until the weekend unfortunately)

Same as you - the fuel pump at the garage cuts out after about a 60 litre delivery. I have found that this isn't so bad if I pump the fuel slower rather than flat-out.

I have also found that withdrawing the nozzle very slightly makes it easier too.

Regarding the fuel gauge accuracy, I have found that all Audis are bad in this respect (my current Audi is my third one)
Half a tank down on the fuel gauge after a long drive, and I can squeeze between 45 and 50 litres of fuel in. You would expect nearer to 35 litres!

I can only assume this is because the tank is not perfectly square in shape (so the guage is not totally accurate, as the level in the tank goes down) and also that there must be some kind of issue with venting/vacuum in the tank.

When my DIS says Zero Miles - I can only ever get 70 litres into my tank.

... I remember seeing a switch somewhere in VAG-COM saying 70 litre tank, and I am wondering if both yourself and I have the wrong setting, so DIS is telling us we need to refuel too early.

I need to check mine (which I won't be able to do until the weekend unfortunately)

Hello Mate,

Good to hear you again.

It looks like I'm not alone with this "problem". I saw 70 liter tank coding when studying ross-tech wiki, when changing MPG to L/100km on my car, but I didn't pay attention, whether it's coded or not. If you are right, it means that the value 256 is added to 4 first digits, and that would explain why fuel gauge shows empty when you consume 60-70 liters of fuel.

I'll connect VAG today and we will see if your theory is right. I hope it's right!!

Because of this issue, I wasn't able to set record of 1000km on one fuel tank as I was scared that I'll run out of fuel on motorway. I did 900km from Dunquerke up to south Berlin ring, but stopped for refueling when reserve light came on and there was 30 km left on the tank. I was surprised at the pump, that I could put 60 liters of fuel when I had first pump stop! At least 15 liters for use which could give me easily 200km more on the tank.
I was cruising from UK to Poland at 150km/h speed for most of the trip and I did 6.6L/100km (42mpg) with 3 people on board and loaded trunk.

Yes, I get between 600 & 625 miles to a tank on a long run (depending upon how fast I drive)

90% of my driving is on the motorway.

I can get an absolute maximum of 73 litres of fuel in - that is from full to the brim (I can see the fuel lapping around the flapper valve at the neck of the filler) to when the DIS is telling me zero miles.

However, if i top up the tank when the needle is on 1/2 tank, I can get about 40 litres in.

1000 kilometres is 621 miles, so that would be about right with your finding too!

Yes, I get between 600 & 625 miles to a tank on a long run (depending upon how fast I drive)

90% of my driving is on the motorway.

I can get an absolute maximum of 73 litres of fuel in - that is from full to the brim (I can see the fuel lapping around the flapper valve at the neck of the filler) to when the DIS is telling me zero miles.

However, if i top up the tank when the needle is on 1/2 tank, I can get about 40 litres in.

1000 kilometres is 621 miles, so that would be about right with your finding too!

in theory it should give me +8.75L more on the gauge. We will see how it will go.

I did some tests on ch29 and 30, by decreasing it to minimum value of 100 (96 is documented as minimal, but it didn't accept the coding), and gauge moved from F to middle of 3/4 and F, so it looks like you have found the proper way of calibrating the gauge reading!

Yesterday I filled up the tank with 70 litres of diesel (63 to the first pump stop). Before filling, it was near reserve bar, but reserve light didn't come on. I'll try to refill again about the same time. It should be more fuel going into the tank than before (circa 9L more).

Ps. I have monochrome DIS, and +1024 coding didn't do anything on my instrument cluster.